1. Field of the Invention
The present invention relates to optical waveguides, or more particularly to lithographically formed single-mode optical waveguides employing organic polymeric materials. The waveguide structure has a low propagation loss.
2. Technical Background
In optical communication systems, messages are transmitted by carrier waves at optical frequencies that are generated by such sources as lasers and light-emitting diodes. There is interest in such optical communication systems because they offer several advantages over conventional communication systems. They have a greatly increased number of channels of communication as well as the ability to transmit messages at much higher speeds than electronic systems using copper wires. This invention is concerned with the formation of light-transmissive optical waveguide devices. The operation of an optical waveguide is based on the fact that when a medium which is transparent to light is surrounded or otherwise bounded by another medium having a lower refractive index, light introduced along the inner medium""s axis is highly reflected at the boundary with the surrounding medium, thus producing a guiding effect.
It is possible to produce polymeric optical waveguides and other optical interconnect devices which transport optical signals in optical circuitry or optical fiber networks. One method used to form an optical device involves the application of standard photolithographic processes. Photopolymers are of particular interest for optical interconnect applications because they can be patterned by photolithographic techniques which are well known in the art. Photopolymers also offer opportunity for simpler, more cost effective manufacturing processes. Lithographic processes are used to define a pattern in a light-sensitive, photopolymer containing layer deposited on a substrate. Among the many known photopolymers, acrylate materials have been widely used as waveguide materials because of their optical clarity, low birefringence and the ready availability of a wide range of monomers.
Planar polymer waveguides typically comprise layers of low loss optical materials of precise indices of refraction. Both step index and gradient index waveguide structures are known in the art. For planar polymer and glass waveguides, in particular, step index structures are most easily achieved through successive coating of materials with differing indices of refraction. Typically, the core has a refractive index which is 0.5% to 2% higher than the clad. The magnitude of this refractive index difference (xcex94n) is set to optimize the performance of the planar waveguides or to match light modes when the transition is made from the planar device to an optical fiber. In reality, most planar waveguide structures have a configuration where the underclad is applied first, followed by application and definition of the core layer, and followed finally by application of the overclad. Due to the height of the core, the overclad typically has a bump on it that can be quite large. This can occur in polymer waveguides in which polymers must be spin cast from a solvent solution due to their high molecular weight and viscosity. It can also occur in silica waveguides in which chemical vapor deposition of the overclad applies a uniform layer on top of the rib. In addition, reactive-ion etching of polymer or glass waveguide cores can result in high propagation losses due to scattering of light caused by rough sidewalls.
Waveguides can be made using photopolymerizable optical materials which can be coated and cured on a substrate. Typically, the materials include mixtures of monomeric and oligomeric components which are blended to provide the correct index of refraction. Mixtures are blended to provide a xcex94n between core and clad, of typically 0.5 to 2 percent. In the photolithography of these curing mixtures, typically a guiding region having an index gradient instead of a step index can be formed in the underclad layer. Also, a region can form at the side and the top of the core in which an index gradient is found instead of a step index. The formation of the gradient index in the region surrounding the core is due to migration of dissimilar chemical components, particularly a monomer component moving from the core layer into the cladding layers. In the region directly under the core, the monomer component can further react during the formation of the core forming an unwanted guiding region within the undercladding layer. When the lower clad region is of about the same thickness as the core, a guiding layer can be formed that penetrates the full thickness of the clad. In extreme cases it can be as intensely guiding as the core itself and allows light to reach the substrate surface. Since the substrates of this invention may be absorbing at optical wavelengths of importance to telecommunications, any portion of the propagating light that reaches the substrate is subject to absorption. Absorption of light by the substrate leads to a severe undesirable polarization-dependent loss of optical power from the propagating signal.
Other attempts have been made in the art to resolve these issues. One potential solution is using a thick undercladding layer to isolate the core from the substrate to prevent this undesirable result. Eliminating the problem to the desired degree, however, requires the use of an impracticably thick undercladding. Another solution includes using a buffer region with an index which is 2% or more lower than the core, wherein the buffer region is below the underclad. Even if monomer diffusion occurs deeply through the underclad and slightly into the buffer, the guiding in the buffer will be greatly suppressed, eliminating most light absorption by the silicon. However, the underclad can still guide light and multimode waveguides with residual polarization effects can still result.
One method of lithographically forming optical elements uses an acrylic photoactive composition which is capable of forming a waveguide material upon polymerization. However, this utilizes polymers with as high a glass transition temperature as possible in order to provide for the greatest operating temperatures. Another method involves the production of waveguides using light polymerizable compositions such as acrylics having a Tg of at least 100xc2x0 C. The foregoing waveguides suffer from undesirably high optical loss.
The invention provides a single-mode optical waveguide fabricated on a substrate wherein the substrate defines a surface. The single-mode optical waveguide comprises a polymeric buffer layer on the surface of the substrate, wherein the buffer layer defining a surface and having an index of refraction nb. A thin, polymeric undercladding layer is on the surface of the buffer layer, wherein the undercladding layer defining a surface and having an index of refraction layer nu. A pattern of a light-transmissive single-mode polymeric core is on the surface of the undercladding layer, wherein the core defines a top surface and sidewalls and wherein the core has an index of refraction nc. A polymeric overcladding layer is on the top surface of the core and on the sidewalls of the core and on a portion of the undercladding layer and having an index of refraction no. The undercladding layer has a thickness of from about 10 percent to about 50 percent of a thickness of the core. The core index of refraction nc is greater than the index of refraction of the overcladding layer no and also greater than the index of refraction of the undercladding layer nu. In the waveguide, xcex94n=ncxe2x88x92no, and the difference between nc and the index of refraction of the buffer nb is at least about 1.5 times xcex94n, and the value of xcex94n is such that it produces a single-mode waveguide at optical communication wavelengths.
The invention also provides a method for forming a single-mode optical waveguide on a surface of a substrate. The method comprises the steps of depositing a polymeric buffer layer onto the surface of the substrate, wherein the buffer layer defines a surface and having an index of refraction nb. One then deposits a polymeric undercladding layer onto the surface of the buffer layer, wherein the undercladding layer defines a surface and has an index of refraction nu. One then deposits a pattern of a light-transmissive, polymeric core onto the surface of the undercladding layer such that the undercladding layer has a thickness of from about 10 percent to about 50 percent of a thickness of the core. The core has a top surface and a pair of sidewalls and an index of refraction nc. One then deposits a polymeric overcladding layer onto the top surface core and onto the sidewalls of the core wherein the overcaldding has an index of refraction no. The core index of refraction nc is greater than the index of refraction of the overcladding layer no and also greater than an index of refraction of the undercladding layer nu; wherein xcex94n=ncxe2x88x92no, and wherein the difference between nc and the index of refraction of buffer nb is at least about 1.5 times xcex94n, and wherein the value of xcex94n produces a single-mode waveguide at optical communication wavelengths.
The invention further provides a method for forming a single-mode optical waveguide on a surface of a substrate. The method comprises the steps of depositing a polymeric buffer layer onto a surface of a substrate, wherein the buffer layer defining a surface and having an index of refraction nb. One then deposits a polymeric undercladding layer onto the surface of the buffer layer, wherein the undercladding layer defining a surface and having an index of refraction nu. One then deposits a photosensitive core layer onto the surface of the undercladding layer such that the undercladding layer has a thickness of from about 10 percent to about 50 percent of a thickness of the core, wherein the photosensitive core layer has an index of refraction nc. One then imagewise exposes the photosensitive core layer to actinic radiation and developing the core layer, thereby removing non-image areas of the core layer and not removing image areas of the core layer thus forming a patterned light-transmissive core on the undercladding layer and partially revealing a portion of the undercladding layer, which core has a top surface and a pair of side walls. One then deposits a polymeric overcladding layer onto the top surface of the core, onto the pair of sidewalls of the core and onto the revealed portions of the undercladding layer. The core index of refraction nc is greater than the index of refraction of the overcladding layer no and also greater than the index of refraction of the undercladding layer nu; wherein xcex94n=ncxe2x88x92no, and wherein the difference between nc and the index of refraction of buffer nb is at least about 1.5 times xcex94n, wherein the value of xcex94n produces a single-mode waveguide at optical communication wavelengths.
It would be desirable to produce optical devices from polymeric materials which have low absorption and scattering loss at application wavelengths, and have precisely controllable refractive indexes for mode and numerical aperture control. Precise refractive index control allows control of mode and numeric aperture and permits fabrication of single-mode waveguides that match single-mode fibers in both cross sectional dimensions and numeric aperture. When the core and cladding materials are comprised of two or more miscible monomers, the index at each layer of a waveguide can be precisely tailored by mixing selected pairs of high index and low index monomers. This property can be used to precisely control the mode of a waveguide and can be used to fabricate large-size single-mode waveguides that match commercial single-mode fibers in both cross sectional dimensions and numeric aperture.
In this invention, a planar waveguide structure is formed in which a buffer, a reduced-thickness underclad, a core, and a normal-thickness overclad are applied to a substrate. Upon the buffer layer, a unusually thin underclad layer is applied and pre-cured. A core layer is applied on top of the reduced-thickness underclad. During the core application and cure, diffusion of low molecular weight, high index of refraction material takes place and increases the index of the underclad. A gradient index is formed through the underclad. The gradient index then sharply falls off with distance into the buffer region. However, the dimension of the reduced-thickness underclad and core are chosen such that optical multimode behavior is frustrated for all potential values of underclad index. In addition, an overcladding is applied, which coats both the sides and the top of the core. A similar diffusion of high index monomer occurs thereby assuring a gradient index around the core. The index profile is found to be now more balanced around the sides and bottom of the waveguide core. According to this invention, clear single-mode performance can be combined with exceptionally low coupling loss due to the improved mode matching with round single core fibers.